Providers of digital video content, audio content, and computer applications, etc. often are reluctant to deliver these items over the Internet or otherwise without effective protection. While the technology exists for providers to deliver content over the Internet, digital content by its very nature is easy to duplicate either with or without the owner's authorization. The Internet allows the delivery of the content from the owner, but that same technology also permits widespread distribution of unauthorized, duplicated content.
Digital Rights Management (DRM) is a digital content protection model that has grown in use in recent years as a means for protecting file distribution. DRM usually encompasses a complex set of technologies and business models to protect digital media, computer applications or other data (all of which is generally referred to herein as “content”) and to provide revenue to content owners.
Many known DRM systems use a storage device, such as a hard disk drive component of a computer, that contains a collection of unencrypted content provided by content owners. The content in the storage device resides within a trusted area behind a firewall. Within the trusted area, the content residing on the storage device can be encrypted. A content server receives encrypted content from the storage device and packages the encrypted content for distribution. A license server holds a description of rights and usage rules associated with the encrypted content, as well as associated encryption keys. (The content server and license server are sometimes part of a content provider system that is owned or controlled by a content provider (such as a studio) or by a service provider.) A playback device or client receives the encrypted content from the content server for display or other use and receives a license specifying access rights from the license server.
Some DRM processes consist of requesting an item of content, encrypting the item with a content key, storing the content key in a content digital license, distributing the encrypted content to a playback device, delivering a digital license file that includes the content key to the playback device, and decrypting the content file and playing it under the usage rules specified in the digital license.
Some digital licenses include time-based rules that control, for example, the expiration date of a license, or the length of time that content may continue to be used by a client device after an initial use of that content. To check such rules, a client or other device must use a clock. However, client system clocks frequently are subject to tampering, thereby making it possible in some circumstances to circumvent time-based rules. Accordingly, improved methods and systems of protection are desirable to accomplish delivery of protected content that is governed by licenses having temporal requirements.